Structure and Properties of Ti-Al Intermetallic Coatings Reinforced with an Aluminum Oxide Filler

Abstract

In this paper, the results of a study of the structure and phase composition of the hot-dip aluminizing coatings formed on the commercially pure titanium surface in AW-6063 aluminum alloy melt after heat treatment at 700 and 850 °C are presented. It is shown that as a result of aluminizing on the titanium surface, a homogeneous coating 30–40 µm thick without defects is formed. The hot-dip aluminizing coating consists of aluminum and the intermetallic compound TiAl₃, located at the boundary with the substrate. Heat treatment results in the formation of a heterogeneous coating structure: its outer layer has a frame-type structure consisting of TiAl₃ particles surrounded by an Al₂O₃ + TiO₂ grid, and the inner continuous layer adjacent to the titanium consists of TiAl₂, TiAl, and Ti₃Al intermetallic layers. Increasing in the heat treatment temperature and/or holding time results in an increase in the thickness of both the outer and boundary layers of the coating. A mechanism for the formation of the coating structure via heat treatment is proposed. The scratch test method was used to evaluate the cohesive and adhesive strength of the coatings, and their scratch hardness was determined, which averaged 200 MPa. It was shown that the coating structure formed during heat treatment at 850 °C ensures higher resistance to cohesive failure.

1. Introduction

Titanium and its alloys are characterized by low heat and wear resistance, which makes it difficult to use them at temperatures above 600 °C under wear conditions. A feature of titanium and its alloys is the presence of a relatively thin oxide film on the surface, which is easily destroyed by friction at high specific loads due to the significantly higher plasticity of titanium compared to the oxide film [1]. The presence of areas of the titanium surface free of oxide film, prone to the formation of bonds, in the case of its friction with other metals, leads to the possibility of seizure in local areas of contact between two surfaces. During friction, oxide films are damaged and peeled off, and new ones are formed in their place. Therefore, a material whose distinctive feature is high corrosion resistance due to surface passivation may prove to be less resistant to friction due to continuous damage and removal of the oxide film. When titanium works in pairs with other metals and alloys, it sticks to the surface of the harder metal and friction then occurs as in a titanium-titanium pair. When titanium works in pairs with a softer material, the soft material is transferred to the titanium surface and friction develops as in the same pair of soft materials.

The wear resistance of titanium alloy can be increased by surface treatment (including laser or electron beam surface alloying, thermal diffusion treatment) or by applying electrolytic, PVD, and cold- or plasma-sprayed coatings [2,3]. The applied coatings must have high adhesion to the base material and ensure effective passivation of the sample surface. At the same time, the methods should not have a negative impact in terms of grain growth and reduction of the mechanical properties of titanium.

The traditional approach to protecting titanium from high-temperature oxidation is to apply a protective coating that ensures the formation of a slowly growing Al₂O₃ oxide on its surface [4]. For this purpose, aluminum coatings are usually applied to the surface of titanium and titanium alloys using spraying [5,6,7,8], hot-dip aluminizing [9,10], pack aluminizing [11,12,13,14] and deposition [15,16], laser synthesis [17,18,19] or self-propagating high-temperature synthesis (SHS) [20,21], mechanical alloying [22], surfacing [23,24], high-temperature thermal diffusion [25], etc.

In [26], an approach is proposed to increase both the heat and wear resistance of aluminized titanium by microarc oxidation, which ensures the formation of a thick layer of aluminum oxide on the surface of the aluminum coating. The authors, by combining hot-dip aluminizing and microarc oxidation technologies, obtained a multilayer coating structure consisting of an Al₂O₃ oxide layer, aluminum, and TiAl₃ intermetallic layer. Studies of the coating properties demonstrated its high hardness, acceptable corrosion resistance, and excellent wear resistance. However, the main problem with such materials is the insufficient bond at the “matrix–reinforcing component” interface and the technological complexity of ensuring uniform distribution of the reinforcing component in the matrix material.

An analysis of the results of studies of liquid-phase interaction in the Ti-Al system [27,28,29] allowed us to conclude that it is promising to obtain heat-resistant coatings on the titanium surface using a complex technology that includes hot-dip aluminizing and subsequent heat treatment at a temperature above the melting point of aluminum. Of all the methods for obtaining aluminum coatings, hot-dip aluminizing is the most economical and effective method, and the reaction interaction of titanium with molten aluminum leads to the formation of intermetallic layers containing rounded intermetallic inclusions of TiAl₃ in an aluminum matrix [9,10,11].

In [10], it is shown that the use of hot-dip aluminizing in combination with subsequent heat treatment at a temperature above the melting point of aluminum leads to the formation of an aluminide coating based on TiAl₃ with a high level of the Al₂O₃ oxide phase distributed throughout the volume of the coating. The implementation of this approach ensures the formation of coatings based on titanium aluminides reinforced with an aluminum oxide filler, the use of which is advisable to increase the high-temperature and tribological properties of the composition.

The aim of this work is to study the influence of temperature–time parameters of heat treatment on the structural, chemical, and phase composition characteristics, as well as the adhesive and cohesive properties of coatings based on titanium aluminides obtained by hot-dip aluminizing of titanium.

2. Materials and Methods

Hot-dip aluminizing of technically pure titanium bulks (VSMPO-AVISMA Corporation, Moscow, Russia) with dimensions of 10 × 10 × 2 mm was carried out by immersing them in a melt of AW-6063 aluminum alloy (RUSAL, Volgograd, Russia). The chemical composition of the initial materials is presented in

Table 1. Chemical composition of commercially pure titanium.

Ti	Fe	Si	N	O	H	C	Others
99.24–99.70	≤0.25	≤0.10	≤0.04	≤0.2	≤0.01	≤0.07	<0.3

and

Table 2. Chemical composition of AW-6063 alloy.

Al	Mg	Si	Mn	Fe	Zn	Cu	Cr	Ti
97.50–99.35	0.45–0.90	0.2–0.6	≤0.10	≤0.35	≤0.10	≤0.10	≤0.10	≤0.10

. To obtain the melt, the aluminum alloy was heated in a SNOL 8.2/1100 furnace (AB UMEGA-GROUP, Utena, Lithuania) to a temperature of 740 °C in a graphite crucible. Titanium bulks were pre-cleaned on sandpaper with a grain size of up to 800 and then degreased. The prepared bulks were immersed in an aluminum alloy bath for 2 min and then removed from the crucible and cooled in air.

Heat treatment was performed in an SNOL 8.2/1100 electric furnace (AB Umega Group, Utena, Lithuania) in the air at 700 and 850 °C. The lower temperature corresponds to the aluminum alloy melting point and the upper temperature does not exceeding the titanium allotropic transformation temperature. The exposure time was up to 20 h.

The cross-sectional microstructures of samples were observed by scanning electron microscopy (SEM–Versa 3D, Thermo Fisher Scientific Inc., Hillsboro, OR, USA) in conjunction with energy-dispersive X-ray spectroscopy (EDS-EDAX Trident XM 4, EDAX, Inc., Mahwah, NJ, USA). In order to perform the microstructure observation and chemical analysis using SEM, the cross-sections of the samples were polished using abrasive papers and then mirror-finished using a diamond slurry with a particle size of 0.5 μm.

A Bruker D8 Advance X-ray diffractometer (XRD—Bruker AXS GmbH, Karlsruhe, Germany), using CuKα radiation (λ = 0.15406 nm), was used to identify the intermetallic phases. To obtain the XRD patterns near the coating–substrate interface, the outer part of the Ti-Al coating was mechanically removed by grinding and finally etched in sodium hydroxide.

The study of the processes of adhesive/cohesive destruction of coatings, as well as the determination of their hardness by scratching, was carried out using a PMT-3M microhardness tester (JSC LOMO, Saint Petersburg, Russia). A Vickers tetrahedral diamond pyramid with an apex angle of 136° was used as an indenter. Scratching was carried out on the surface of the metallographic sample with the pyramid edge so that the diagonal of the pyramid base was located along the direction of indenter movement. The indenter was moved from the substrate to the coating perpendicular to the interface. The horizontal driving force was applied through a microscrew to the rotating platform of the microhardness tester. The scratches 500 μm long were applied in the plane of the metallographic specimen from the Ti substrate through the coating and beyond its limits. The uniform movement speed of the table was 20 μm/s. The vertical load on the indenter varied within 0.2–5 N. To increase the reliability of the results, measurements were carried out on 3 samples obtained under the same conditions.

The critical distance (L_c) was used as a parameter characterizing the cohesive destruction of the coating, i.e., the distance from the beginning of the conical widening of the scratch to the surface of the coating (Figure 1). The criterion for adhesive destruction was delamination at the coating–substrate interface.

Metallographic studies of the coating thickness and scratch parameters were carried out using an optical microscope (Olympus BX-61, Olympus Corporation, Tokyo, Japan) with the AnalySIS Pro 3·2 software (Soft Imaging System GmbH, Münster, Germany). For each parameter, 10 measurements were taken.

The value of scratch hardness (H, MPa), which characterizes the resistance to destruction, was determined using the following Formula (1) [30]:

H = (0.3782 × F)/b²,

(1)

where F is the normal force, N, and b is the scratch width, mm.

3. Results and Discussion

3.1. As-Aluminized State

Metallographic studies (Figure 2) showed that as a result of the interaction of titanium with molten aluminum alloy, a continuous coating with a thickness of ~30–40 µm without defects (pores, cracks) is formed on its surface. A more detailed analysis of the obtained SEM images shows that the structure of the coating has a predominantly homogeneous structure. At the coating–substrate interface, there is a layer of lighter inclusions in relation to the matrix layer, comprising a round shape with a size of 250–400 nm.

Based on the obtained EDS data (Figure 2,

Table 3. Point EDS analysis results (see Figure 2).

Element	Content, at.%
	1	2	3
Al	100	-	83.35
Ti	-	100	16.65

), it can be concluded that the outer layer consists of a crystallized aluminum matrix. The inclusions at the coating–substrate interface have a composition of ~83–84 at.% Al and ~16–17 at.% Ti. According to the Al-Ti phase diagram [31], aluminum and titanium can form the following intermetallic compounds: TiAl₃, TiAl₂, TiAl, and Ti₃Al. The TiAl and Ti₃Al intermetallic layers have wide homogeneity regions. At ~750 °C, they extend in the ranges of 48–63.3 and 22–35 at.% Al for TiAl and Ti₃Al, respectively. The solubility of aluminum in α-titanium at this temperature is 13 at.%. Titanium dissolves in aluminum only slightly. The TiAl₃ and TiAl₂ intermetallics have very narrow regions of existence at all temperatures. Therefore, the formation of a solid solution of Al (Ti) is unlikely even taking into account the effect of nonequilibrium crystallization. The indicated composition of inclusions probably corresponds to the compound TiAl₃, which is the richest in aluminum. On the other hand, Kattner [32] showed that the TiAl₃ phase in a wide temperature range has a minimum free energy of formation compared to the TiAl and Ti₃Al phases formed by a single-stage reaction between molten aluminum and solid titanium. The TiAl₂ phase has lower free energies of formation compared to TiAl₃, but its formation can only occur through a series of reactions in the solid–liquid and/or solid state when consuming TiAl as the initial phase, so the formation of TiAl₂ is excluded for thermodynamic reasons.

Since the interpretation of the point EDS analysis results is difficult due to the very small sizes of the individual intermetallic inclusions (neighboring areas with a different chemical composition may be involved in the region of generation of characteristic radiation), XRD analysis was additionally carried out at different coating depths from the surface of the coating (Figure 3a) and in the immediate vicinity of the boundary with titanium (Figure 3b).

The XRD patterns contained only Al, Ti, and TiAl₃ intermetallic (PDF2 #03-065-5174) peaks. TiAl₃ is represented by a tetragonal cell with the parameters a = 3.854 Å and c = 8.584 Å.

3.2. Heat-Treated State

Heat treatment results in the formation of a heterogeneous (instead of the initial homogeneous) structure of the hot-dip aluminizing coating. Visually, the cross-section of the coatings after heat treatment (Figure 4) can be divided into two layers. The outer layer, which occupies most of the coating, consists of rounded light inclusions, separated by a dark grid. At the coating–substrate boundary, there is a thinner continuous layer. The total coating thickness increased with increasing heat treatment temperature and holding time. The most intensive growth in thickness is observed during holding for up to five hours, then the growth slows down (Figure 5). As a result of heat treatment, the thickness of the coating layer increases almost 3–4 times compared to the initial state after aluminizing.

EDS analysis showed that at 700 °C in the studied time range (up to 20 h), the phase composition of the outer layer of the coating does not change and consists of particles of the intermetallic compound TiAl₃ separated by an oxide. The oxides are aluminum rich (55–64 at.%), while the oxygen content in them does not exceed 25 at.%, which is probably due to the low thickness of the analyzed layer. In the middle part of the coating, Fe-rich (~13 at.%) light inclusions were found (Figure 6), the formation of which is due to the presence of iron in the titanium and AW-6063 alloy. Based on the literature data [33], it can be concluded that these inclusions correspond to the ternary compound τ₃—Ti₈Fe₃Al₂₂ or a solid solution based on FeAl₃ or TiAl₃ aluminides.

XRD analysis allowed us to reliably identify the formation of TiAl₃ aluminide in the coating (Figure 7). It should be noted that the diffraction pattern obtained from the coating surface after heat treatment at 700 °C for 20 h, in addition to the intense peaks of the TiAl₃ phase (PDF2#03-065-5174), contains peaks of the same compound with the cell parameters a = 3.880 Å and c = 33.847 Å (PDF2#00-026-0038). The formation of a similar phase, which was designated as Ti₈Al₂₄, was observed by the authors [27] during long-term (120 h) low-temperature (585 °C) annealing of titanium–aluminum diffusion couples; they were also the first to describe its crystal structure. The authors of [34] noted the formation of Ti₈Al₂₄ in cast and powder alloys of the Al75Ti25 composition after annealing for 23 days at 640 °C and 6 days at 620 °C. Formation of this phase during room temperature mechanical grinding was reported for the first time in [35]. As for the composition of the oxide phases, despite the presence of a dark grid on the observed microstructures (Figure 4), no oxide peaks were found on the diffraction pattern of the coating after heat treatment at 700 °C for 20 h, either on the surface or in the depths of the coating.

Increasing the heat treatment temperature to 850 °C leads to the intensification of diffusion processes and an increase in the oxide volume fraction in the outer layer of the coating (Figure 8). In addition, the formation of TiAl₂ aluminide is observed at the boundaries of TiAl₃ particles after 5 h of holding (Figure 9a,b). After 20 h of holding at 850 °C (Figure 9c,d), in addition to TiAl₃ and TiAl₂, inclusions corresponding to the stoichiometric composition of the compound τ₃—Ti₈Fe₃Al₂₂ and the solid solution FeAl₃(Ti) are fixed in the middle part of the coating. According to [36], up to 6.5 at.% Ti can dissolve in FeAl₃ (Fe₄Al₁₃) compounds at 800 °C. The formation of iron-rich phases, as mentioned earlier, is most likely associated with the initial composition of the titanium and aluminum alloy.

A higher temperature promotes the saturation of the coating with oxygen, as indicated by a significant increase in the oxygen concentration in the oxide to ~50 at.% (Figure 9). XRD analysis (Figure 10) revealed that, along with aluminum oxide, the coating contains titanium oxide TiO₂, indicating the formation of a mixture of Al₂O₃ and TiO₂ oxides in the outer layer of the coating during heat treatment. It should be noted that the most thermodynamically probable event at high temperatures is the formation of Al₂O₃ instead of TiO₂, since the Al₂O₃ has a much more negative formation energy [37]. At the initial stages of heat treatment, Al₂O₃ is formed when oxygen interacts with aluminum-rich areas, and, subsequently, as the temperature and/or time increases, part of the titanium is oxidized and TiO₂ forms. This assumption is confirmed by the presence of Ti-rich oxide areas in the coating structure (Figure 9). In addition to peaks from the oxide phases, the diffraction patterns of the coating contain TiAl₃ and TiAl₂ aluminide peaks. In the surface layer of the coating, the Ti(Fe_0.5Al_0.5)₂ phase with a hexagonal cell with parameters a = 4.9649 Å and c = 8.0593 Å is additionally identified, which is a ternary Laves phase with the MgZn₂ structure type. According to [38], its chemical composition corresponds to equiatomic Ti33.3Fe33.3Al33.4.

After heat treatment at 700 °C at the boundary with titanium, a continuous diffusion zone with a thickness of ~5–10 μm is formed, consisting of several layers representing the aluminides TiAl₂, TiAl, and Ti₃Al (as they approach titanium) (Figure 11). Its phase composition is formed as a result of solid-phase reactions occurring in the following sequence: TiAl₃ → TiAl₂ → TiAl → Ti₃Al. According to the calculation performed in [10], the energies of formation of TiAl, TiAl₂, and TiAl₃ at 700 °C are, respectively, equal to −21,105, −33,135, and −30,264 kJ/mol. The difference in the formation energy between TiAl₂ and TiAl₃ indicates that TiAl₂ can grow into a separate layer only with an increase in temperature or time of heating to overcome the interphase energy barrier compared to the temperature–time parameters of hot-dip aluminizing.

When the heat treatment temperature is 850 °C, this leads to an increase in the thickness of the continuous diffusion zone to 15–20 μm (Figure 12). Its phase composition remains unchanged: TiAl₂, TiAl, and Ti₃Al (as it approaches titanium). The thickness of the continuous diffusion zone changes mainly due to the increase in the thickness of the TiAl and Ti₃Al layers. It can be expected that with an increase in the temperature and/or time of heat treatment, the presence of a concentration gradient will lead to the formation of a solid solution of aluminum in titanium Ti(Al) at the Ti₃Al–Ti boundary.

3.3. Mechanism of Coating Structure Formation

The mechanism of the appearance of a continuous diffusion zone at the interface between solid titanium and molten aluminum during aluminizing apparently does not differ from the first stages of the formation of intermetallic layers at the interface between two solid metals according to the model [39], which includes mutual diffusion of the contacting metals at different rates, the emergence of locally supersaturated solid solutions around defects, the formation of the first centers of a new phase in defective areas with an increased concentration of the diffusing element, transverse growth of the centers of the intermetallic phase along the plane of the joint, closure, and normal growth (perpendicular to the interface) of the continuous intermetallic TiAl₃ layer. During subsequent heat treatment, growth of the TiAl₃ interlayer and sequential formation of the TiAl₂, TiAl, and Ti₃Al phases occurs due to the presence of a concentration gradient.

The formation of a two-phase Al + TiAl₃ structure of the outer coating layer can be explained by the destruction of a continuous TiAl₃ aluminide layer under the action of stresses arising from differences in the volumes of reacted titanium and the intermetallic compound formed at the coating–substrate boundary [40]. Stresses in the TiAl₃ layer increase as its thickness increases. A brittle continuous TiAl₃ layer can undergo crack formation and fragmentation after reaching critical thickness and stress. Cracking can be intensified by the Rehbinder effect, which facilitates dispersion during contact between a solid and a liquid under tensile stress conditions [41,42]. The separated fragments expose a new reaction surface, along which the intermetallic synthesis reaction proceeds more actively. The heat of the exothermic reaction of TiAl₃ formation increases the temperatures of the reaction products and the aluminum melt, which leads to the occurrence of a temperature gradient and convective flows. Melt flows in the ascending direction transport the separated TiAl₃ fragments from the reaction zone. After some time, depending on the temperature–time modes of heat treatment and the initial coating thickness, the separated TiAl₃ fragments reach the free surface of the sample. The continuous Al₂O₃ film on the aluminum alloy surface is destroyed, which leads to accelerated penetration of oxygen deep into the coating due to a significant increase in the free surface and the appearance of channels of facilitated oxygen diffusion. As a result, the “residual” aluminum, which was not spent on the formation of TiAl₃ and did not diffuse into titanium, is transformed into aluminum oxide (Al₂O₃). As the temperature and/or holding time increases, some of the titanium oxidizes and TiO₂ is formed.

After cooling from the heat treatment temperature, a frame-type structure of the coating is formed from TiAl₃ particles surrounded by an Al₂O₃ + TiO₂ grid. Such a structure allows for the ideal implementation of the Charpy principle, widely known in tribology, according to which materials consisting of a viscous matrix (TiAl₃ intermetallic) and high-strength inclusions (Al₂O₃ + TiO₂ oxides filler) have optimal tribological characteristics [43].

Previously, we studied diffusion processes in aluminum + nickel, aluminum + nichrome explosively-welded layered composites after their heat treatment above the melting point of aluminum [44,45], as well as the effect of high-temperature heating on the structure and properties of hot-dip coatings on Ni-Cr and Fe-Ni-Cr alloys [46], but the formation of such frame-type structures was not observed.

3.4. Scratch Tests

The scratch tests allowed us to evaluate the adhesive and cohesive strength of the coatings. After heat treatment at 700 °C, the coating has an extended zone without oxide inclusions on the titanium side, which has a higher hardness, as indicated by the narrowing of the scratch. In this zone, cohesive cracks are observed, diverging to the sides from the scratch. The transition to the zone with oxide inclusions is accompanied by a widening of the scratch and the formation of larger cracks, leading to coating partial chipping. Cracks indicating adhesive destruction are not observed (Figure 13). After heat treatment at 850 °C, the intermetallic (TiAl₃) and oxide (Al₂O₃ + TiO₂) components are uniformly distributed over the outer layer of the coating. The transition from the titanium substrate to the coating, despite the high hardness of the aluminides and oxides, does not practically change the scratch width. The indenter passage over the coating is accompanied by the chipping of individual inclusions of the intermetallic and oxides and their subsequent removal from the scratch. Adhesive and cohesive cracks in the coating are not observed up to a load of 3 N (Figure 13).

An analysis of the obtained graphical dependencies of the scratch hardness (Figure 14a) and the critical distance L_c (Figure 14b) indicates a strong dependence of the studied parameters on the applied load. At low load values up to 0.5–1 N, deviations in the scratch hardness values from the steady-state values at a higher load are observed, which, in the case of the substrate, may be associated with a measurement error at a low loading level. A significant increase in the scratch hardness of the coatings at low loads is due to the presence of a continuous intermetallic layer of TiAl₃ in the coating structure after heat treatment at 700 °C. An increase in the load leads to the destruction of the TiAl₃ intermetallic layer and a sharp decrease in scratch hardness. High scratch hardness values at low loads in coated samples after heat treatment at 850 °C can be explained by the insufficiency of such a load to destroy the oxide–intermetallic interphase bonds. With increasing load, the interphase bonds are broken, and the hardness decreases to ~200 MPa.

As can be seen from Figure 14b, an increase in the load is accompanied by an increase in the critical distance, which characterizes the tendency of the coating to cohesive failure. For the coating structure formed after heat treatment at 850 °C, lower values of the critical distance are characteristic. Analysis of the obtained data indicates that in hot-dip aluminizing coatings, the mechanism of the so-called viscous failure is realized, in which peeling and chipping of the coatings can occur only under very high loads, or in the case of weak adhesion [47]. The main mechanism for this system is cohesive failure.

4. Conclusions

Hot-dip aluminizing of commercially pure titanium in AW-6063 aluminum alloy allows the formation of a defect-free homogeneous coating that is 30–40 µm thick on the entire surface of the sample, consisting of aluminum with a small amount of the TiAl3 intermetallic, located mainly at the boundary with the substrate.

Heat treatment of the hot-dip aluminizing coating at temperatures that ensure the melting of aluminum and do not exceed the allotropic transition temperature in titanium leads to the formation of its heterogeneous structure. The outer layer of the coating has a frame-type structure consisting of TiAl₃ particles surrounded by an Al₂O₃ + TiO₂ grid. A layered structure consisting of layers of the intermetallic compounds TiAl₂, TiAl, and Ti₃Al is formed at the boundary with titanium. Increasing the heat treatment temperature from 700 °C to 850 °C leads to an increase in the volume fraction of oxides in the outer part of the coating and the growth of a continuous diffusion zone at the boundary with titanium. A mechanism for the formation of the coating structure is proposed.

The resistance to the formation of cohesive cracks of hot-dip aluminizing coatings on the titanium surface, assessed by scratch tests, increases after heat treatment at 850 °C.